Breast cancer can be stratified into several distinct subtypes, with each subtype expressing differentiation proteins that are primarily associated with either luminal or basal cells of the normal mammary gland. Importantly, stratification of breast cancer into these subtypes is correlated with clinical outcome. In order to investigate the molecular and genetic pathways that give rise to breast cancer diversity, it is critical that in vitro and in vivo models more accurately embody the spectrum of this disease. We hypothesize that the diversity of breast cancer reflects the heterogeneity of normal breast epithelial cells from which tumors arise, and that several cell lineages including stem, progenitor and differentiated cells can acquire the regenerative qualities of cancer stem cells, leading to different tumor subtypes. To investigate the relationship between a tumor's cellular origin and breast cancer, we will adapt our recently developed lentiviral-mediated transduction method in order to induce oncogene expression in different mouse and human mammary lineages. Specific Aim 1 seeks to establish whether novel mouse-modeling methodology could produce different tumor subtypes by expanding the spectrum of cell populations targeted by oncogenes in reconstituted mammary outgrowths. Specific Aim 2 will investigate whether tumor-initiating cells derived from different tumor subtypes exhibit unique biological qualities since they retain molecular attributes associated with their cellular origin. In Specific Aim 3 we will again employ cell-type targeting techniques in order to assess whether oncogenesis within different human breast cell populations would enable better recapitulation of cancer subtypes observed in patients. Taken together, the studies proposed in this grant application should answer several important questions about the association of cellular differentiation and breast cancer heterogeneity, in addition to establishing new methodology that may better model breast cancer diversity.